1. Field of the Invention
The present invention relates to a visible light receiving method and an apparatus of the method which are used for visible light communication. In particular, the present invention relates to a visible light receiving method and an apparatus of the method which are suitably usable in a portable terminal such as a personal digital assistant, a mobile telephone which is equipped with a digital camera, a digital video camera, or the like.
2. Description of Related Art
In recent years, visible light communication using a visible light as a communication medium has been developed, and a visible light communication apparatus that performs visible light communication by use of a visible light such as an illuminating ray has been proposed in JP No. 2008-283446 A and the like.
With respect to this conventional visible light communication apparatus, a visible light communication apparatus which superimposes a transmitting signal on a visible light to send it operates so as to generate a transmitting signal by performing modulation by the 4-level PPM method (pulse-position modulation) onto an information signal (pulse signal) to be transmitted, to apply the transmitting signal to a light source using an LED to drive the LED, and superimpose the transmitting signal on a visible light irradiated from the LED, to transmit it.
Further, in this visible light communication apparatus, in order to effectively use an ordinary use light source for illumination as a light source of the visible light communication apparatus, at the time of applying a PPM signal to the LED light source for illumination, the LED is continuously lit during a period of a digital value of “0” which is relatively long, and the LED is operated to blink on and off at a shorter frequency by a sub-carrier carrier signal during a period of a digital value of “1” which is short in time, thereby securing necessary illuminance for illumination at the time of transmission in visible light communication.
On the other hand, in recent years, portable terminals such as mobile telephones have become remarkably popular, and most people carry portable terminals with them on a daily basis. Further, because a digital camera available for video recording is normally mounted in this type of portable terminal, users live in an environment where they can routinely photograph with a camera.
Therefore, conventionally, in JP No. 2011-55288 A, there has been proposed a visible light communication apparatus which photographs a visible light irradiated from a light source for illumination serving as a light source of the visible light communication apparatus that performs visible light communication, that is, an illuminating ray for visible light communication on which an information signal to be transmitted is superimposed with a camera of a portable terminal or the like, and analyzes the photographed image data to extract a data bit string contained in the image data, and demodulates information signal data transmitted from the data bit string.
However, because the above-described visible light communication apparatus photographs the light source of the visible light communication apparatus with, for example, a light source for illumination serving as a visible light source with a digital camera or a video camera, that is, directly photographs a luminous visible light irradiated from the light source for illumination (light from the light source) with the camera, the exposure of the camera is logically automatically adjusted for the entire viewing surface. Therefore, an image of the light source on the photographed image goes blank so as to be photographed in an overexposed state.
Therefore, even if the image data photographed by the camera is subjected to image processing, it is extremely difficult to precisely extract visible light transmission data contained in the image data in an overexposed state even for an image processing technology of a microcomputer operating at high speed.
Further, when photographing a visible light, even in the case where a photographed image is not brought into an overexposed state, because a contour and the like of the image are contained as noise in visible light transmission data, it is difficult to precisely extract the visible light transmission data from the image data.
In addition, a general-purpose digital camera or video camera photographs a moving image at 16 frames to 30 frames per second, and calculates luminance values of image data of the respective frames, to capture the luminance value data as time-series data for each frame number, and further calculates the maximum luminance value and the minimum luminance value in the calculated luminance value data, to extract a data bit string of receiving data on the basis of a predetermined threshold value.
For this reason, the camera photographs image data at an extremely long time axis or with a long time interval that is 1/16 seconds to 1/30 seconds, and extracts a data bit string of transmission data on the basis of the luminance value data for each frame. Therefore, if a visible light image is photographed at a speed of 30 frames/sec., data of one sample are acquired with respect to one frame, and the one sample data are extracted for about 33 milliseconds, that is, the extraction of receiving data is at extremely low speed.
The visible light communication apparatus usually performs multilevel PPM modulation such as 4-level PPM modulation onto a transmitting signal, to generate a transmitting signal, and superimposes the transmitting signal on a visible light irradiated from an LED to transmit it. However, in this case, a frequency of a transmitted pulse signal generated by the PPM modulation is about 9.6 KHz, for example, in the case where a data transmission speed is 4.8 kbps, and the signal has a single pulse width of a data bit string of about 0.1 milliseconds, that is an extremely short time axis. This pulse width is an extremely short time as compared with the respective frame times of an image photographed by a camera.
Therefore, in the visible light receiving apparatus using a conventional camera, in addition to the fact that it is extremely difficult to extract receiving data from an image including a contour of an object photographed by the camera, or image data in an overexposed state by image processing, it is impossible to extract received data at high speed because transmission data transmitted through visible light communication (for example, in the case of a data transmission speed of 4.8 Kbps, a signal at a frequency of about 9.6 kHz) is acquired at a speed of 1 sample for one frame.